The present invention relates to the inspection of solid bodies, such as generator retaining rings, and particularly indirect determination of the profile of such a body along a given axis.
In industry, there is frequently a need to inspect machine parts which are subject to failure due to wear or other causes. For example, generator shaft retaining rings, which are mounted on the end of a generator shaft to retain windings in place, must be periodically inspected to identify any flaws which may have developed therein. The need to inspect such rings has gained increasing importance because the useful life of a large generator can depend on the condition of its retaining ring. Typically, cracks develop in the ring surface which contacts the generator windings, and these cracks frequently start at locations where a transition occurs in the inside diameter of the ring. If such cracks reach a critical size, the ring may rupture, resulting in the potential for extensive damage to the generator and surrounding equipment.
In order to forestall such an occurrence, many utilities perform periodic inspections of the generator retaining rings. The most common inspection method is an ultrasonic examination in which a high frequency ultrasonic signal is directed radially from the ring outer circumference toward the ring inner circumference. By monitoring the ultrasonic energy travel time, together with a precise knowledge of the ring profile, small cracks emanating from the inner circumference can be detected.
The success of such a procedure depends on an accurate knowledge of the ring profile, and such knowledge is not always available at the generator location. Removal of a ring to measure its profile is not acceptable for economic reasons and design drawings of the ring are not always available.